Everyone knows greenheads as a nuisance, but according to the North Shore Greenhead Fly Control Project, they can also take a bite out of Essex County's economy, affecting real estate values, beach use, golfing and outdoor recreation.

But, as experts point out, the tiny pests are somewhat of a necessary evil. Thousands of their larvae are eaten by fish, shorebirds and small animals each year. While people may not enjoy them, a strong greenhead presence is actually a sign of a healthy ecosystem.

"We definitely wouldn't want to get rid of them," said Nancy Pau, a wildlife biologist at the Parker River National Wildlife Refuge. "The fact that we have them on Plum Island in this salt marsh system is an indication that it is really healthy and intact."

With the current abundance of hot and humid weather preferred by greenheads, Pau said the North Shore's fly season is in full swing,and probably won't die down until the end of the month, with some straggling insects hanging around through the first week of August.

Until then, U.S. Fish and Wildlife Service park ranger Joan Abare has some suggestions for those looking to enjoy summer days on the beach while keeping greenhead bites to a minimum.

She said the flies are attracted to cool, shadowy spaces, and tend to prey on people wearing dark colors. They also prefer the salty skin of ocean swimmers, and are attracted to scents; wearing strong-smelling perfumes, deodorants and hair products will also draw biting female flies.

Bug spray is not an effective method of deterring greenheads. Abare and Pau agree the best way to keep them off is to wear clothing that covers as much skin as possible.

"Most of the calls we get this time of year are about greenheads," Abare said. "We advise people to wear lighter colors and to towel off after they get out of the water,"

Abare also pointed out the greenheads' intolerance of windy conditions.

"They might seek shelter from the wind under umbrellas or other structures that block the wind," she said.

Strangely enough, greenheads do not bother everyone equally. While on any given day, some people may constantly dodge and swat the flies, others are able to go virtually unbothered by them.

"They pick and choose, so if you bring someone that tastes better than you do, you'll be fine," Abare said.